Display device and method of driving display device

ABSTRACT

A display device including a pixel, a drive transistor including a first and a second terminal and a gate, a first switching element controlling a connection between the second terminal and the gate, a storage capacitor of a second terminal connected to the gate, a second switching element controlling a connection between the storage capacitor and a first signal line, a third switching element arranged in parallel with the second switching element and controlling a connection between the storage capacitor and a second signal line, a fourth switching element controlling a connection between the drive transistor and a first power supply line, a light emitting element connected with the drive transistor and a second power supply line, a fifth switching element controlling a connection between the drive transistor and the light emitting element, a sixth switching element controlling a connection between the drive transistor and a third power supply line.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2015-219335 filed on Nov. 9,2015, the entire contents of which are incorporated herein by reference.

FIELD

The present invention is related to a circuit structure of a pixel in adisplay device and a circuit structure which operates by compensatingthe characteristics of a transistor which drives a display element.

BACKGROUND

Display devices are being developed in which a light emitting elementusing an organic electroluminescence material is arranged in each pixeland an image is displayed by individually controlling the light which isemitted. A light emitting element includes a structure in which a layerhaving an organic electroluminescence material (also called an “organiclayer”) is sandwiched between a pair of electrodes, one beingdistinguished as an anode electrode and the other as a cathodeelectrode. A display device formed with a pixel using such a lightemitted element includes a pixel electrode in which one of theelectrodes is arranged in each pixel, and a common electrode in whichthe other electrode is applied with a voltage common to a plurality ofpixels. A display device displays an image by individually controllingthe voltage of a pixel electrode in each pixel and setting the voltageof a common electrode as a fixed voltage.

A light emitting element arranged in each pixel is connected to atransistor. Since the transistor drives the light emitting element, itis called as a drive transistor. A drive transistor arranged in eachpixel in a pixel region arranged with pixels is ideally preferred tohave uniform characteristics. However, a plurality of drive transistorsactually arranged in a pixel region has variation in electricalcharacteristics. At times, there is variation in a threshold voltage ofa drive transistor and due to this, even when the same gate voltage isapplied to a plurality of drive transistors, a problem arises whereby adrain current fluctuates. As a result, it is known that variation in theluminosity of a light emitting element occurs which degrades the displayquality of an image. In order to solve such problems, a technology hasbeen developed which compensates for a threshold voltage of a drivetransistor arranged in each pixel (for example, see U.S. PatentApplication No. 2006/0023551 specification, and U.S. Patent ApplicationNo. 2007/00262931 specification).

SUMMARY

A display device in an embodiment according to the present inventionincludes a pixel including a drive transistor, a first switchingelement, a storage capacitor, a second switching element, a thirdswitching element, a fourth switching element, a light emitting element,a fifth switching element and a sixth switching element, the drivetransistor including a first terminal, a second terminal and a gatewhich controlling a current flowing between the first terminal and thesecond terminals, the first switching element controlling a connectionbetween the second terminal and the gate of the drive transistor, thestorage capacitor including a first terminal and a second terminal, thesecond terminal of the storage capacitor connected to the gate of thedriving transistor, the second switching element controlling aconnection between the first terminal of the storage capacitor and afirst signal line supplied with a data signal, the third switchingelement arranged in parallel with the second switching element andcontrolling a connection between the first terminal of the storagecapacitor and a second signal line supplied with a reference signal, thefourth switching element controlling a connection between the firstterminal of the drive transistor and a first power supply line suppliedwith a first voltage, the light emitting element including a firstterminal and a second terminal, the first terminal of the light emittingelement connected with the second terminal of the drive transistor andthe second terminal of the light emitting element connected with asecond power supply line supplied with a second voltage lower than thefirst voltage, and the fifth switching element controlling a connectionbetween the second terminal of the drive transistor and the firstterminal of the light emitting element, the sixth switching elementcontrolling a connection between the first terminal of the drivetransistor and a third power supply line supplied with a third voltagelower than the first voltage and higher than the second voltage.

A method for driving a display device in an embodiment according to thepresent invention, a pixel including a drive transistor, a firstswitching element, a storage capacitor, a second switching element, athird switching element, a fourth switching element, a light emittingelement, a fifth switching element and a sixth switching element, thedrive transistor including a first terminal, a second terminal and agate which controlling a current flowing between the first terminal andthe second terminals, the first switching element controlling aconnection between the second terminal and the gate of the drivetransistor, the storage capacitor including a first terminal and asecond terminal, the second terminal of the storage capacitor connectedto the gate of the driving transistor, the second switching elementcontrolling a connection between the first terminal of the storagecapacitor and a first signal line supplied with a data signal, the thirdswitching element arranged in parallel with the second switching elementand controlling a connection between the first terminal of the storagecapacitor and a second signal line supplied with a reference signal, thefourth switching element controlling a connection between the firstterminal of the drive transistor and a first power supply line suppliedwith a first voltage, the light emitting element including a firstterminal and a second terminal, the first terminal of the light emittingelement connected with the second terminal of the drive transistor andthe second terminal of the light emitting element connected with asecond power supply line supplied with a second voltage lower than thefirst voltage, and the fifth switching element controlling a connectionbetween the second terminal of the drive transistor and the firstterminal of the light emitting element, the sixth switching elementcontrolling a connection between the first terminal of the drivetransistor and a third power supply line supplied with a third voltagelower than the first voltage and higher than the second voltage, themethod including in a reset time period the first switching element andthe fifth switching element are turned ON, the fourth switching elementis turned OFF, a reference voltage is applied to the storage capacitorfrom the second signal line via the third switching element, and a thirdvoltage is applied to the other input/output terminal of the drivetransistor from the third power supply line, in a signal programming andoffset cancel time period the first switching element and the fourthswitching element are turned ON, the fifth switching element is turnedOFF, a voltage based on a data signal from the first signal line isapplied to the other terminal of the storage capacitor via the secondswitching element, and the first voltage is applied from the first powersupply line and the third power supply line to the first terminal of thedrive transistor, and in a light emitting time period the firstswitching element is turned OFF, the fourth switching element and thefifth switching element are turned ON, a reference voltage is applied tothe first terminal of the storage capacitor via the third switchingelement, and the light emitting element emits light by a current flowingfrom the first power supply line via the drive transistor.

A display device in an embodiment according to the present inventionincludes a first pixel, a second pixel, a third pixel, and fourth pixel,each of the first to the fourth pixel including a drive transistor, afirst switching element, a storage capacitor, a second switchingelement, a third switching element, a fourth switching element, a lightemitting element and a fifth switching element, the drive transistorarranged including a first terminal, a second terminal and a gate whichcontrolling a current flowing between the first terminal and the secondterminals, the first switching element controlling a connection betweenthe second terminals and the gate of the drive transistor, the storagecapacitor including a first terminal and a second terminal, the secondterminal of the storage capacitor connected to the gate of the drivingtransistor, the second switching element controlling a connectionbetween the first terminal of the storage capacitor and a first signalline supplied with a data signal, the third switching element arrangedin parallel with the second switching element and controlling aconnection between the first terminal of the storage capacitor and asecond signal line supplied with a reference signal, the fourthswitching element controlling a connection between the first terminal ofthe drive transistor and a first power supply line supplied with a firstvoltage, the light emitting element including a first terminal and asecond terminal, the first terminal of the light emitting elementconnected with the second terminal of the drive transistor and thesecond terminal of the light emitting element connected with a secondpower supply line supplied with a second voltage lower than the firstvoltage, and the fifth switching element controlling a connectionbetween the second terminal of the drive transistor and the firstterminal of the light emitting element. Each of the drive transistorincluded in the first to the fourth pixels connected to a third powersupply line connected with a sixth switching element controlling theapplication of a third voltage lower than the first voltage and higherthan the second voltage to the first terminal, the first and the secondpixel, and the third and the fourth pixel are arranged in a rowdirection, the first and the third pixel, and the second and the fourthpixel are arranged in a column direction, the fourth switching elementis shared between the first to fourth pixels, and the second switchingelement and the third switching element are shared between the first andthird pixels, and the second and fourth pixels arranged in a columndirection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective diagram for explaining a structure of a displaydevice related to one embodiment of the present invention;

FIG. 2 is a diagram for explaining a structure of a display devicerelated to one embodiment of the present invention;

FIG. 3 is a diagram showing a circuit structure of a pixel in a displaydevice related to one embodiment of the present invention;

FIG. 4 is a timing chart for explaining a driving method of a displaydevice related to one embodiment of the present invention;

FIG. 5 is a circuit diagram for explaining a driving method of a displaydevice related to one embodiment of the present invention;

FIG. 6 is a circuit diagram for explaining a driving method of a displaydevice related to one embodiment of the present invention;

FIG. 7 is a circuit diagram for explaining a driving method of a displaydevice related to one embodiment of the present invention;

FIG. 8 is a circuit diagram for explaining a driving method of a displaydevice related to one embodiment of the present invention;

FIG. 9 is a circuit diagram showing a circuit structure of a pixel in adisplay device related to one embodiment of the present invention;

FIG. 10 is a timing chart for explaining a driving method of a displaydevice related to one embodiment of the present invention;

FIG. 11 is a diagram showing a circuit structure in a pixel region of adisplay device related to one embodiment of the present invention;

FIG. 12 is a timing chart for explaining a driving method of a displaydevice related to one embodiment of the present invention; and

FIG. 13 is a cross-sectional diagram showing a structure of a part of apixel that can be applied to one embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will hereinafter be described withreference to the drawings. However, the present invention can beimplemented in many different modes, and is not to be interpreted asbeing limited to contents of description of embodiments illustratedbelow. While the width, the thickness, the shape, and the like of eachof portions may be more schematically indicated than those in an actualmode to make the description clearer, this is only one example, and isnot to limit the interpretation of the present invention. In the presentspecification and the drawings, detailed description may be omitted, asneeded, by assigning similar elements to those described above withreference to the already described drawing the same reference signs.

In the present specification, when a member or region exists “on (orunder)” another member or region, this includes not only a case wherethe member or region exists just above (or just below) the other memberor region but also a case where the member or region exists above (orbelow) the other member or region, i.e., includes a case where anothercomponent is included between the member or region above (below) theother member or region and the other member or region except asotherwise limited.

In the present specification, electrically connected refers to a statein which a voltage path or a current path is formed. For example, afirst transistor and a second transistor are electrically connected evenwhen a third transistor is arranged between the first transistor and thesecond transistor. In addition, in the present specification, aconnection is sometimes used to mean electrically connected.

In the case of an n channel type drive transistor, an offset canceloperation which compensates a threshold voltage of a drive transistorchanges a source voltage by setting the source voltage to a floatingstate, and setting the voltage between the gate and source of the drivetransistor to a threshold voltage Vth. A voltage of a data signalcompensated with a threshold voltage is stored in a storage capacitorconnected to the gate by writing a data voltage of a signal to bedisplayed in the pixel in this state.

In order to set a threshold voltage of a drive transistor between a gateand source (or gate and drain), it is necessary to apply a certain drainvoltage to the drain of the drive transistor and cause a drain currentto flow. However, when there is influence from wire resistance of a wirewhich applies a drain voltage, a problem occurs where variation isproduced in the offset cancel operation itself and a threshold voltageVth is not sufficiently compensated. For example, a problem occurswhereby a luminosity distribution is produced between pixels in ahorizontal direction in a pixel region where wires which apply a drainvoltage extend in a horizontal direction.

In addition, when a voltage drop occurs due to the influence of wireresistance, a problem occurs when offset cancel is not sufficientlyperformed and a threshold voltage of a drive transistor is notsufficiently compensated. This type of problem is particularly apparentwhen attempting to achieve miniaturization of wires using due to highdefinition of a pixel or large scale pixel regions (large screens).

According to one embodiment of the present invention described below, adisplay device and driving method of a display device are exemplifiedwhich can overcome this type of problem.

1. Structure of a Display Device

FIG. 1 shows a summary of a display device 100 related to one embodimentof the present invention using a perspective view. The display device100 includes a pixel region 102 arranged with a plurality of pixels 104,a scanning line drive circuit 106, and a data line drive circuit 108.There are arranged in a first substrate 110. A terminal part 109 inputwith a signal is arranged in the first substrate 110. A wiring substrate111 is connected to the terminal part 109. The wiring substrate 111 isalso called a flexible circuit substrate (FPC substrate) and is used forconnection with external devices which provide signal to the displaydevice 100. The pixel region 102 is covered by a sealing member so as tonot be exposed to the air. Although the sealing member 112 may be hardsubstrate such as a glass substrate, the sealing member 112 may also bean organic resin film substrate or an organic resin layer.

FIG. 2 shows a structure of a wire arranged in the pixel region 102 inthe display device 100. FIG. 2 shows an example in the case where pixels104 are arranged in m number of rows and n number of columns in thedisplay region 102. In this case, the number of pixels in the displayregion 102 is m×n pixels. Furthermore, although FIG. 2 shows an examplewhere the pixels 104 are arranged in a square, the present invention isnot limited to this arrangement and the pixels may also have a deltaarrangement or other arrangement shape.

A display element is arranged in a pixel 104. Each pixel 104 is drivenby a scanning line drive circuit 106 and data line drive circuit 108. Inone embodiment of the present invention, a light emitting element isused as a display element. For example, an organic electroluminescenceelement which uses an organic electroluminescence material is used asthe light emitting element.

The scanning line drive circuit 106 is connected with first scanningsignal lines TG1˜TGm, second scanning signal lines IG1˜IGm, thirdscanning signal lines BG1˜BGm, and fourth scanning signal lines EG1˜EGm.The scanning line drive circuit 106 is formed including a shift registerand a control signal is sequentially output to each scanning line. Inthe pixel region 102, the first scanning signal lines TG1˜TGm, secondscanning signal lines IG1˜IGm, third scanning signal lines BG1˜BGm, andfourth scanning signal lines EG1˜EGm are arranged corresponding to eachrow of a pixel. In addition, power supply lines PVD1˜PVDm correspondingto each row are connected from each scanning line drive circuit 106.Details of the power supply lines are described herein.

The data line drive circuit 108 is connected with first signal linesVS1˜VSn and second signal lines VR1˜VRn. The data line drive circuit 108outputs a data signal for displaying an image to the first signal linesVS1˜VSn. In addition, the data line drive circuit 108 outputs areference signal to the second signal lines VR1˜VRmn. In the pixelregion 102, a first signal line Vs and second signal line VR arearranged corresponding to each column.

Furthermore, in the explanation given above, the reference symbols “m”and “n” are integers and correspond to the number of each signal linearranged in the pixel region 102.

2-1. Circuit Structure of a Pixel (Example 1)

FIG. 3 shows a circuit structure of a pixel 104. A pixel 104 of thedisplay device 100 related to one embodiment of the present invention isformed including a first switching element TCT, a second switchingelement ICT1, a third switching element ICT2, a fourth switching elementBCT3, a fifth switching element EMT, a light emitting element EMD, and astorage capacitor element CS.

As is explained below, a pixel 104 in the display device 100 related toone embodiment of the present invention includes a first conduction typetransistor arranged with the first switching element TCT, secondswitching element ICT1, and fifth switching element EMT, and aconduction type transistor opposite to the first conduction typetransistor arranged with the third switching element ICT2 and fourthswitching element BCT3.

A drive transistor DRT is arranged with at least two input/outputterminals (first terminal and second terminal) and a gate as a controlterminal for controlling a current flowing between the input/outputterminals. The drive transistor DRT is arranged with a source and drainas input/output terminals. An insulation gate type field effecttransistor is used for the drive transistor DRT for example and a thinfilm transistor is favorably used as one type of insulation gate typefield effect transistor.

In one embodiment of the present invention, a p channel type transistoris used for the drive transistor DRT. One input/output terminal (firstterminal) of the drive transistor DRT is arranged on a first powersupply line PVH side, and the other terminal (second terminal) isarranged on a second power supply line PVS side. In the explanationbelow, for convenience the input/output terminal on the first powersupply line PVH side in the drive transistor DRT is given as the sourceand the input/output terminal on the second power supply line PVS sideis given as the drain.

A first voltage PVDD_H is applied to the first power supply line PVH anda second voltage PVSS is applied to the second power supply line PVS.Here, the first voltage PVDD is assumed to be a higher voltage than thesecond voltage PVSS.

The light emitting element EMD is a two terminal (first terminal andsecond terminal) element which exhibits diode characteristics. The lightemitting element EMD is biased in a forward direction and emits lightwhen a voltage above a light emitting threshold voltage is applied. Thelight emitting intensity of the light emitting element EMD changes inproportion to an increase or decrease in the amount of current in arange of normal operation. One terminal (first terminal, anode forexample) of the light emitting element EMD is electrically connected toone terminal (drain) of the drive transistor DRT, and the other terminal(second terminal, cathode for example) is electrically connected to thesecond supply line PVS.

The storage capacitor element CS is connected to the gate of the drivetransistor DRT. In addition, the first switching element TCT is arrangedbetween the gate and drain of the drive transistor DRT. The firstswitching element TCT controls the connection between one of theinput/output terminals and the gate of the drive transistor DRT.

Furthermore, a switching element forms a conduction state when ON andforms a non-conducting state when OFF. A switching element is formedusing a transistor for example.

The ON and OFF state of the first switching element TCT is controlled bya control signal (amplitude VGH/VGL) of the first scanning signal lineTG. When the first switching element TCT is ON, the gate and drain areelectrically connected and the drive transistor DRT becomes a diodeconnected state. In one embodiment of the present invention, the firstswitching element TCT is formed by an n channel type transistor.

Furthermore, a signal with an amplitude VGH as a control signal providedby a scanning signal line has a voltage level which switches an nchannel type transistor ON and a p channel type transistor OFF, and asignal with an amplitude VGL (or amplitude VGL1) has a voltage levelwhich switches an n channel type transistor OFF and a p channel typetransistor ON.

On terminal (second terminal) of the storage capacitor element CS iselectrically connected with the gate of the drive transistor DRT and theother terminal (first terminal) is connected with one terminal of thesecond switching element ICT1 and third switching element ICT2. Thesecond switching element ICT1 and third switching element ICT2 arearranged in parallel and one terminal on the side input with a signalare both electrically connected with the other terminal of the storagecapacitor element CS.

The other terminal which is the signal input side of the secondswitching element ICT1 is electrically connected with the first powersupply line VS which is supplied with a data signal. The other terminalwhich is the signal input side of the third switching element ICTs iselectrically connected with the second power supply line VR which issupplied with a reference signal. An ON/OFF operation of the secondswitching element ICT1 and third switching element ICT2 is controlled bya control signal (amplitude VGH/VGL) of the second scanning signal lineIG.

The second switching element ICT1 and third switching element ITC2perform an exclusive operation. That is, when the same control signal(amplitude VGH/VGL) is applied, one of the second switching element ICT1or third switching element ITC2 is switched ON and the other is switchedOFF. A circuit formed from the switching element ICT1 and thirdswitching element ITC2 can be viewed as a selection circuit whichoutputs one signal with respect to two input signals. In order torealize this type of operation, in the case where the second switchingelement ICT1 is formed by a first conductive type transistor (forexample, n channel type transistor), the third switching element ITC2 isformed by a second conductive type transistor (for example, p channeltype transistor). The second conductive type is the reverse of the firstconductive type. FIG. 3 shows the case where the second switchingelement ICT1 is formed by a n channel type transistor and the thirdswitching element ICT2 is formed by a p channel type transistor.

The first terminal (source) of the drive transistor DRT is electricallyconnected with the first power supply line PVH via the fourth switchingelement BCT3. When the fourth switching element BT3 is ON, the source ofthe drive transistor DRT is in a conducting state with the first powersupply line PVH and a first voltage PVDD_H is applied.

In addition, the third power supply line PVD is electrically connectedto the source of the drive transistor DRT. The third voltage PVDD_L isapplied to the third power supply line PVD. Furthermore, the thirdvoltage PVD_L is lower than the first voltage PVDD_H, higher than thesecond voltage PVSS and is a voltage between the first voltage PVDD_Hand second voltage PVSS.

The third power supply line PVD is connected with the sixth switchingelement BCT2. The third power supply line PVD is applied with the thirdvoltage PVDD_L via the sixth switching element BCT2. The sixth switchingelement BCT2 controls the timing when the third voltage PVDD_L isapplied to the third power supply line PVD. The sixth switching elementBCT2 does not have to be arranged in the pixel 104 or pixel region 102,for example, the sixth switching element BCT2 may be arranged in aregion of the scanning line driving circuit 106.

An ON/OFF operation of the sixth switching element BCT2 is controlled bya control signal (amplitude VGH/VGL) of the third scanning signal lineBG. That is, an ON/OFF operation of the fourth switching element BCT3and the sixth switching element BCT2 is controlled by the same controlsignal (amplitude VGH/VGL) of the third scanning signal line BG. Here,the fourth switching element BCT3 and the sixth switching element BCT2perform an exclusive operation. That is, when the same control signal(amplitude VGH/VGL) is applied, one of the fourth switching element BCT3or sixth switching element BTC2 is switched ON and the other is switchedOFF. In order to realize this type of operation, in the case where thefourth switching element BCT3 is formed by a first conductive typetransistor, the sixth switching element BTC2 is formed by a conductivetype transistor the reverse of the first conductive type. FIG. 3 showsthe case where the fourth switching element BCT3 is formed by a pchannel type transistor and the sixth switching element BCT2 is formedby a n channel type transistor.

Both the fourth switching element BCT3 and sixth switching element BCT2are controlled by a control signal (amplitude VGH/VGL) of the thirdscanning signal line BG. Since the first power supply line PVH and thirdpower supply line PVD are both connected to the source of the drivetransistor DRT, it is not preferable that different voltage levels areapplied from these two power supply lines. Therefore, the fourth switchelement BCT3 and sixth switching element BCT2 are configured to performan exclusive operation. That is, when the fourth switching element BCT3is formed by a first conductive type transistor, the sixth switchingelement BTC2 is formed by a second conductive type transistor thereverse of the first conductive type. FIG. 3 shows the case where thefourth switching element BCT3 is formed by a p channel type transistorand the sixth switching element BTC2 is formed by a n channel typetransistor. In this way, even when the same control signal (amplitudeVGH/VGL) is applied from the third power supply line BG to the fourthswitching element BCT3 and sixth switching element BTC2, it is possibleto ensure that both switching elements are not ON at the same time.

A current which flow to the light emitting element EMD is controlled bythe drive transistor DRT. A fifth switching element EMT is arrangedbetween one terminal (anode) of the light emitting element EMD and oneterminal (drain) of the drive transistor DRT. The fifth switchingelement EMT controls an electrical connection between one terminal(anode) of the light emitting element EMD and one terminal (drain) ofthe drive transistor DRT. An ON/OFF operation of the fifth switchingelement EMT is controls by a control signal (amplitude VGH/VGL) of thefourth scanning signal line EG. FIG. 4 shows an example where the fifthswitching element is formed by a n channel type transistor. When thefifth switching element EMT is ON, a drain current flows from the drivetransistor DRT to the light emitting element EMD and light is emitted.

The drive transistor DRT controls the amount of current which flows tothe light emitting element EMD, and the fifth switching element EMTfunctions as a switching element for controlling the timing when thelight emitting element EMD emits light. By arranging a fifth switchingelement EMT having such as function, it is possible to control the lightemitting timing for each pixel.

In the pixel 104 shown in FIG. 3, the first power supply line PVH isarranged corresponding to a column direction with respect to at leastthe arrangement of a pixel. In addition, the first power supply line PVHmay be arranged in a mesh shape in a row direction and a columndirection with respect to the arrangement of a pixel. In this way, it ispossible to apply the first voltage PVDD_H uniformly to each pixelwithin a pixel region. Thereby it is possible to achieve a uniform imagedisplay.

2-2. Operation of a Display Device (Example 1)

Next, the operation of the pixel shown in FIG. 3 is explained. Thedisplay device 100 is driven including at least three time periods, areset time period, a signal writing and offset cancel time period and alight emitting time period. An idle time period during which a waveformof a control signal applied to a scanning line shifts may be includedbetween the reset time period and signal writing and offset cancel timeperiod.

FIG. 4 shows a timing chart of a first scanning signal line TG, secondscanning signal line IG, third scanning signal line BG and fourthscanning signal line EG. In addition, a data signal Vsig (herein avoltage based on a data signal is also called “data voltage Vsig”) whichis applied to a first signal line VS and a reference signal Vref (hereina voltage based on reference signal is also called “reference voltageVref”) which is applied to a second signal line VR synchronized with acontrol signal of these scanning lines are also shown.

In a reset time period, a storage capacitor element is discharged andcharged with a reference voltage Vref is charged. The state of a pixelcircuit at this time is shown in FIG. 5. Furthermore, FIG. 5 shows thefirst switching element TCT, second switching element ICT1, thirdswitching element ICT2, fourth switching element BCT3, fifth switchingelement EMT and sixth switching element BCT2 using switch symbols forsimplification.

A control signal (amplitude VGH/VGL) of the first scanning signal lineTG becomes a high level voltage (amplitude VGH) and the first switchingelement TCT is turned ON. A control signal (amplitude VGH/VGL) of thesecond scanning signal line IG is a low level voltage (amplitude VGL)and the second switching element ICT1 is turned OFF, the third switchingelement ICT is turned ON, and the other terminal of the storagecapacitor element CS is connected with the second signal line VR and isapplied with a reference voltage Vref.

A control signal (amplitude VGH/VGL) of the fourth scanning signal lineEG is a high level voltage (amplitude VGH) and the fifth switchingelement EMT is turned ON. One terminal of the storage capacitor elementCS becomes connected with the second power supply line PVS via the firstswitching element TCT, fifth switching element EMT and light emittingelement EMD. In this way, the charge (charge charged in the previousframe) charged in the storage capacitor element CS is discharged.

The storage capacitor element CS is discharged until a voltage of oneterminal of the light emitting element EMD (terminal on the sideconnected with the drain of the drive transistor DRT) converges to avoltage (PVSS+Vem) obtained by adding a threshold voltage Vem of thelight emitting element EMD to the second voltage PVSS. The controlsignal (amplitude VGH/VGL) of the third scanning signal line is a highlevel voltage (amplitude VGH), the third switching element BCT3 isturned OFF, the sixth switching element BCT2 is turned ON, and a thirdvoltage PVDD_L is applied from the third power supply line PVD to thesource of the drive transistor DRT. In this way, the source and drain ofthe drive transistor DRT are reset to the same voltage, namely the thirdvoltage PVD_L. In a reset time period, since the storage capacitorelement CS is connected to the second signal line VR and charged with areference voltage Vref, the data of a data signal written in theprevious frame is deleted.

In an idle time period, the storage capacitor element CS is separatedfrom the second power supply line PVS, and a first voltage PVDD_H isapplied to the source of the drive transistor DRT. The state of a pixelcircuit at this time is shown in FIG. 6. A control signal (amplitudeVGH/VGL) of the first scanning signal line TG becomes a low levelvoltage (amplitude VGL), and the first switching element TCT is turnedOFF. A control signal (amplitude VGH/VGL) of the second scanning signalline IG is a low level voltage (amplitude VGL), the second switchingelement ICT1 is turned OFF, the third switching element ICT2 is turnedON, and a state in which a reference voltage Vref is applied from thesecond signal line VR to one terminal of the storage capacitor elementCS is maintained.

A control signal (amplitude VGH/VGL) of the fourth scanning signal lineEG changes to a low level voltage (amplitude VGL) from a high level andthe first switching element TCT changes from ON to OFF. In this way, theconnection between the drive transistor DRT and the light emittingelement EMD is broken. Following this, a control signal (amplitudeVGH/VGL) of the third scanning signal line BG changes to a low levelvoltage (amplitude VGL) from a high level, the sixth switching elementBC2 is turned OFF, and the third switching element is turned ON. In thisway, a first voltage PVDD_H is applied from the first power supply linePVH to the source of the drive transistor DRT.

In a signal writing and offset cancel time period, an operation forcompensating a threshold voltage Vth of the drive transistor DRT isperformed, and a data signal is written to the storage capacitor elementCS. The state of a pixel circuit at this time is shown in FIG. 7. Acontrol signal (amplitude VGH/VGL) of the second scanning signal line IGchanges from a lower level to a high level voltage (amplitude VGH), thesecond switching element ICT1 is turned ON, the third switching elementICT2 is turned OFF, and a data voltage Vsig is applied from the firstsignal line VS to the other terminal of the storage capacitor elementCS.

Following this, a control signal (amplitude VGH/VGL) of the firstscanning signal line TG becomes a high level voltage (amplitude VGH),and the first switching element TCT is turned ON. In this way, the drainand gate of the drive transistor DRT are in a conducting state. Acontrol signal (amplitude VGH/VGL) of the third scanning signal line BGis a low level voltage (amplitude VGL), the third switching element BCT3is turned ON, and the sixth switching element BCT2 is turned OFF. Inthis way, a first voltage PVDD_H is applied from the first power supplyline PVH to the source of the drive transistor DRT.

A control signal (amplitude VGH/VGL) of the fourth scanning signal lineEG is an initial low level voltage (amplitude VGH), and the fifthswitching element EMT is in an OFF state. In this way, the drain of thedrive transistor DRT converges to a voltage (PVDD_H−Vth) obtained byreducing a threshold voltage Vth from the first voltage PVDD_H. Since adata voltage Vsig is applied from the first signal line VS to thestorage capacitor element CS, the voltage between both terminals of thestorage capacitor element CS becomes Vsig−(PVDD_H−Vth). At this time,since the fifth switching element EMT is OFF, a voltage above a lightemitting threshold voltage is not applied to the light emitting elementEMD to obtain a non-light emitting state.

In a light emitting period, light is emitted at an intensity dependingon the amount of a drain current flowing from the drive transistor DRTto the light emitting element EMD. The stat of a pixel circuit at thistime is shown in FIG. 8. A control signal (amplitude VGH/VGL) of thefirst scanning signal line TG changes from a high level to a low levelvoltage (amplitude VGL), and the first switching element TCT is turnedto OFF. A control signal (amplitude VGH/VGL) of the second scanningsignal line IG changes from a high level to a low level voltage(amplitude VGL), and the second switching element ICT1 is turned to OFF,the third switching element ICT2 is turned to ON, and a referencevoltage Vref is applied to the other terminal of the storage capacitorelement CS. In this way, the voltage of the storage capacitor elementbecomes Vref−Vsig+(PVDD_H−Vth).

A control signal (amplitude VGH/VGL) of the third scanning signal lineBG is a high level (amplitude VGH), the fourth switching element BCT3 isturned to ON, and the sixth switching element BCT2 is turned to OFF. Inthis way, the first voltage PVDD_H is applied from the first powersupply line PVH to the source of the drive transistor DRT. In addition,a control signal (amplitude VGH/VGL) of the fourth scanning signal lineEG changes from a low level to a high level voltage (amplitude VGH), andthe fifth switching element EMT is turned to ON. A drain currentcontrolled by the gate voltage of the drive transistor DRT flows and thelight emitting element EMD emits light. Since the substantial gatevoltage of the drive transistor DRT becomes Vref−Vsig, any influence ofthe threshold voltage Vth is cancelled.

By providing the display device 100 related to one embodiment of thepresent invention with a reset time period in which the first switchingelement TCT and the fifth switching element EMT are ON, the fourthswitching element BCT3 is turned OFF, a reference voltage Vref isapplied from the second signal line VR via the third switching elementICT2 to the storage capacitor element CS, and a third voltage PVDD_L isapplied to the other input/output terminal of the drive transistor DRTfrom the third power supply line PVD, a signal writing and offset canceltime period in which the first switching element TCT and the fourthswitching element BCT3 are ON, the fifth switching element EMT is turnedOFF, a voltage based on a data signal Vsig is supplied from the firstsignal line VS via the second switching element ICT1 to the otherterminal of the storage capacitor element CS, and a first voltage PVDD_His applied to the other input/output terminal of the drive transistorDRT from the first power supply line PVH, and a light emitting timeperiod in which the first switching element TCT is OFF, the fourthswitching element BCT3 and fifth switching element EMT are turned ON, areference voltage Vref is applied from via the third switching elementICT2 to the other terminal of the storage capacitor element CS, and acurrent flows via the drive transistor DRT from the first power supplyline PVH and the light emitting element emits light, thereby it ispossible to provide a display without receiving the effects of avariation in a threshold voltage of a drive transistor DRT. In addition,the display device 100 can display an image with no variation inluminosity in a surface (especially in a horizontal direction) of thepixel region 102.

By applying the first voltage PVDD_H which is a high voltage to thesource of the drive transistor DRT and turning the fifth switchingelement EMT OFF, a voltage on the drain side of the drive transistor DRTconverges to a voltage obtained by reducing a threshold voltage Vth fromthe first voltage PVDD_H within an offset cancel time period forobtaining the threshold voltage Vth and securely perform an offsetcancel operation. By performing an offset cancel operation using thefirst voltage PVDD_H which is a high voltage, it is possible to securelycompensate for a threshold voltage of the drive transistor DRT even inthe case where the number of pixels increases and an offset cancel timeperiod allowed within one frame time period is shortened. Furthermore,by arranging power supply lines in a mesh shape, it is possible torelieve the effects of a drop in voltage due to wire resistance of apower supply line and reduce offset cancel variation within the pixelregion 102. Furthermore, since the fifth switching element EMT is OFF, alarge current does not flow via a power supply line within an offsetcancel time period, the effects of a drop in voltage due to wireresistance are relieved, and it is possible to reduce offset cancelvariation within the pixel region 102.

The first power supply line PVH applies the first voltage PVDD_H to eachpixel within a light emitting time period. That is, the light emittingelement EMD is supplied with a drive current from the first power supplyline PVH via the drive transistor DRT. By arranging the power supplylines in a mesh shape as described above, the effects of a drop involtage due to wire resistance are relieved and a variation in lightemitting intensity within the pixel region 102 is reduced.

2-3. Modified Example 1

As in the pixel 104 b shown in FIG. 9, in addition to the sixthswitching element BCT2, a seventh switching element BCT1 may beconnected in parallel to the third power supply line PVD and the firstvoltage PVDD_H may be applied. That is, a power supply voltage appliedto the third power supply line PVD can be switched and control of thisswitching may be performed by the sixth switching element BCT2 andseventh switching element BCT1 arranged in a drive circuit. One terminalof the sixth switching element BCT2 and seventh switching element BCT1is electrically connected with the third power supply line PVD. Theother terminal which is the signal input side of the sixth switchingelement BCT2 is applied with the third voltage PVDD_L, and the otherterminal which is the signal input side of the seventh switching elementBCT2 is connected with the first voltage PVDD_H.

Here, the sixth switching element BCT2 and seventh switching elementBCT1 operate exclusively. That is, when the same control signal(amplitude VGH/VGL) of the third scanning signal line BG is applied, oneof the sixth switching element BCT2 and seventh switching element BCT1is turned ON and the other is turned OFF. In order to realize this typeof operation, in the case where the sixth switching element BCT2 isformed by a first conductive type transistor, the seventh switchingelement BTC1 is formed by a conductive type transistor the reverse ofthe first conductive type. FIG. 3 shows the case where the sixthswitching element BCT2 is formed by a n channel type transistor and theseventh switching element BCT1 is formed by a p channel type transistor.

The fourth switching element BCT3, sixth switching element BCT2 andseventh switching element BCT1 are all controlled by a control signal(amplitude VGH/VGL) of the third scanning signal line BG. Since thethird power supply line PVD and first power supply line PVH are bothconnected to the source of a drive transistor DRT, it is not preferablethat voltages with different levels are applied from these power supplylines. In the present embodiment, while the sixth switching element BCT2is turned ON when a third voltage PVDD_L is applied, the fourthswitching element BCT3 and seventh switching element BCT1 which areturned ON when a first voltage PVDD_H is applied are formed by differentconductive type transistors. In this way, it is possible to ensure thatboth switching elements are not turned ON at the same time even when thesame control signal (amplitude VGH/VGL) is applied to the fourthswitching element BCT3 and sixth switching element BCT2 from the thirdscanning signal line BG. FIG. 3 shows an example in which the fourthswitching element BCT3 and seventh switching element BCT1 are formedusing a p channel type transistor and the sixth switching element BCT2is formed using a n channel type transistor.

As is shown in FIG. 3, a drive transistor DRT is electrically connectedwith a third power supply line PVD which extends in a row direction, anda first power supply line PVH which extends in a column direction via aswitching element. That is, the third power supply line PVD extending ina row direction is connected to a source of the drive transistor DRT viathe seventh switching element BCT1, and the first power supply line PVHis connected with the source of the drive transistor DRT via the fourthswitching element BCT3. In this way, in an offset cancel time period, afirst voltage PVDD_H is applied from the first power supply line PVH andthird power supply line PVD.

That is, it is possible to operate the pixel 104 b shown in FIG. 9 sothat by connecting the seventh switching element BCT1 in parallel andapplying the first voltage PVDD_H to the third power supply line PVD, inthe signal writing and offset cancel time period, the first switchingelement TCT and fourth switching element BCT3 are turned ON, the fifthswitching element EMT is turned OFF, a voltage based the data signalVsig is supplied from the first signal data line VS via the secondswitching element ICT1 to the other terminal of the storage capacitorelement CS, and the first voltage PVDD_H is applied to the otherinput/output terminal of the drive transistor DRT from the first powersupply line PVH and third power supply line PVD, and in a light emittingtime period, the first switching element TCT is turned OFF, the fourthswitching element BCT3 and fifth switching element EMT are turned ON, areference voltage Vref is applied via the third switching element ICT2to the other terminal of the storage capacitor element CS, a currentflows from the first power supply line PVH and third power supply linePVD via the drive transistor DRT and the light emitting element EMDemits light.

The first power supply line PVH and third power supply line PVD applythe first voltage PVDD_H to each pixel in a light emitting time period.That is, the light emitting element EMD is supplied with a drive currentfrom the first power supply line PVH and third power supply line PVD viathe drive transistor DRT. By arranging the power supply lines in a meshshape as described above, the effects of a drop in voltage due to wireresistance are relieved and a variation in light emitting intensitywithin the pixel region 102 is reduced.

2-4. Modified Example 2

After an offset cancel time period which compensates for a thresholdvoltage of a drive transistor DRT, a low level voltage VGL1 of a firstscanning signal line TG is conventionally been required to be a very lowlevel voltage in order to turn the first switching element TCT OFF.However, such a low level voltage VGL1 creates a large reverse biasstate in the first switching element TCT in a light emitting timeperiod, and a problems sometimes occurs where a leak current flows fromthe drain of the drive transistor DRT. As a result, a problem occurswhereby the drain current of the drive transistor DRT increases andluminosity of the light emitting element EMD increases. That is, it isfeared that a problem may occur in which light spot defects are producedin a pixel region 102.

FIG. 10 shows an example in which an intermediate voltage VGL2 is addedin addition to a high level voltage VGH and a low level voltage VGL1,and a control signal of the first scanning signal line TG drives a firstswitching element using the three level signals. After the signalwriting and offset cancel time period, a voltage of the first scanningsignal line TG is set to a lower voltage VGL2 than a high level voltageVGH and higher than the low level voltage VGL1, thereby, the gate of thefirst switching element TCT is applied with a higher voltage than anormal OFF state. In this way, in a light emitting time period, areverse bias state of the first switching element TCT is relieved and itis possible to reduce a leak current. Furthermore, the low level voltageVGL2 is a voltage which can maintain an OFF state of the first switchingelement TOT.

By driving the first switching element TCT using three level waveformsas is shown in FIG. 10, it is possible to continue to compensate for athreshold voltage of a drive transistor DRT and prevent the generationof light spots in a light emitting time period.

3-1. Circuit Structure of a Pixel (Example 2)

In the pixel 104 shown in FIG. 3, the fourth switching element BCT3which controls the connection between the first power supply line PVHand drive transistor DRT can be shared between a plurality of pixels.That is, it is possible to share the fourth switching element BCT3 whichcontrols the connection with the first power supply line PVH betweenpixels adjacent in a row direction and/or a column direction.

FIG. 11 shows an example in which the fourth switching element BCT3 isshared between four pixels (first pixel 104_11, second pixel 104_12,third pixel 104_21 and fourth pixel 104_22) arranged in a row directionand column direction. A drive transistor DRT11, light emitting elementEMD 11, storage capacitor element CS11, first switching element TCT11and fifth switching element EMT11 are included in the first pixel104_11. The same is true for the second pixel 104_12, third pixel 104_21and fourth pixel 104_22.

The second switching element ICT11 and third switching element ICT21which are connected with the other terminal of the storage capacitorelement CS11 of the first pixel 104_11 are arranged to be shared withthe third pixel 104_21. This is also the same for the second switchingelement ICT12 and third switching element ICT22 with respect to thesecond pixel 104_12 and fourth pixel 104_22. That is, the secondswitching element ICT11 which controls the connection between thestorage capacitor element CS11, storage capacitor element CS21 and firstsignal line VS1, and the third switching element ICT21 which controlsthe connection with the second signal line VR1 can be shared between aplurality of pixels. In this way, by sharing the second switchingelement ICT11 and third switching element ICT21, and the secondswitching element ICT12 and third switching element ICT22 between pixelsadjacent in a column direction, it is possible to substantially reducethe number of transistors per pixel.

The first power supply line PVH is connected to the four pixels (firstpixel 104_11, second pixel 104_12, third pixel 104_21 and fourth pixel104_22) via the fourth switching element BCT3 and the third power supplyline PVD is connected to the four pixels via the sixth switching elementBCT2. By arranging the fourth switching element BCT3 to be sharedbetween a plurality of pixels, it is possible to substantially reducethe number of transistors per pixel. This type of structure is useful itterms of miniaturizing pixels and achieving high definition.

Furthermore, as explained referring to FIG. 3, the seventh switchingelement BCT1 may be connected in parallel with the sixth switchingelement BCT2 and the first voltage PVDD_H can be applied to the thirdpower supply line PVD. In this way, it is possible to achieve furtheruniformity in a luminosity distribution in a horizontal direction.

3-2. Operation of a Display Device (Example 2)

FIG. 12 shows a timing chart for explaining the operation of the fourpixels (first pixel 104_11, second pixel 104_12, third pixel 104_21 andfourth pixel 104_22) shown in FIG. 11.

In a reset time period, a control signal (amplitude VGH/VGL) of thefirst scanning signal line TG1 and a control signal (amplitude VGH/VGL)of the second scanning signal line TG2 become a high level voltage(amplitude VGH), and the first switching element TCT11, first switchingelement TCT12, first switching element TCT21 and first switching elementTCT22 are turned ON.

A control signal (amplitude VGH/VGL) of the second scanning signal lineIG is a low level voltage (amplitude VGL), the second switching elementICT11 and the second switching element ICT12 are turned OFF, the thirdswitching element ICT21 and the third switching element TCT22, areturned ON, one end of the storage capacitor element CS11 and storagecapacitor element CS21 is connected with the second signal line VR1, oneend of the storage capacitor element CS12 and storage capacitor elementCS2 is connected with the second signal line VR2 and a reference voltageVref is applied.

A control signal (amplitude VGH/VGL) of the fourth scanning signal lineEG is a high level voltage (amplitude VGH) and the fifth switchingelement EMT11 to fifth switching element EMT22 are switched ON. In thefirst pixel 104_11, the other end of the storage capacitor element CS11is in a state connected with the second power supply line PVS via thefirst switching element TCT11, fifth switching element EMT11 and lightemitting element EMD11. This state is the same for the second pixel104_12, third pixel 104_21 and fourth pixel 104_22. In this way, thecharge charged in the storage capacitor element CS of each pixel isdischarged. The discharge of a storage capacitor element CS in eachpixel in a reset time period is the same as that explained using FIG. 4.Since a storage capacitor element CS in each pixel is connected to thesecond signal line VR and charged with a reference voltage Vref, thedata of a data signal written in the previous frame is deleted.

In an idle time period, a control signal (amplitude VGH/VGL) of thefirst scanning signal line TG1 and a control signal (amplitude VGH/VGL)of the second scanning signal line TG2 become a low level voltage(amplitude VGL), and the first switching element TCT11, first switchingelement TCT12, first switching element TCT21 and first switching elementTCT22 are turned OFF.

In addition, in the first pixel 104_11, the storage capacitor CS11 isseparated from the third power supply line PVD, and the first voltagePVD_H is applied to the source of the drive transistor DRT11. A controlsignal (amplitude VGH/VGL) of the first scanning signal line TG1 becomesa low level voltage (amplitude VGL), and the first switching elementTCT11 is turned OFF. A control signal (amplitude VGH/VGL) of the secondscanning signal line IG remains as a low level voltage (amplitude VGL),the second switching element ICT11 is turned ON, the third switchingelement ICT21 is turned OFF, and a reference voltage Vref is appliedfrom the second signal line VR1 to one terminal of the storage capacitorelement CS11. A control signal (amplitude VGH/VGL) of the fourthscanning signal line EG changes from a high level to a low level voltage(amplitude VGL), and the fifth switching element EMT in each pixelchanges from ON to OFF. Following this, a control signal (amplitudeVGH/VGL) of the third scanning signal line BG changes from a high levelto a low level voltage (amplitude VGL), the fourth switching elementBCT3 is turned ON, and the sixth switching element BCT2 is turned OFF.This operation is also the same for the other pixels. In this way, thefirst voltage PVDD_H is applied to the source of a drive transistor DRTin each pixel.

In the first pixel 104_11 in the signal writing and offset cancel timeperiod, a control signal (amplitude VGH/VGL) of the second scanningsignal line IG changes from a low level to a high level voltage(amplitude VGH), the second switching element ICT11 is turned ON, thethird switching element ICT21 is turned OFF, and a data voltage Vsig1 isapplied from the first signal line VS1 to one terminal of the storagecapacitor element CS11. A control signal (amplitude VGH/VGL) of thefirst scanning signal line TG1 becomes a high level voltage (amplitudeVGH), and the first switching element TCT11 is turned ON. In this way,the drain and gate of the drive transistor DRT11 is in a conductingstate. A control signal (amplitude VGH/VGL) of the third scanning signalline BG is a high level voltage (amplitude VGH), the third switchingelement BCT3 is turned ON, and the sixth switching element BCT2 isturned OFF. In this way, the first voltage PVDD_H is applied from thefirst power supply line PVH to the source of the drive transistor DRT11.A control signal (amplitude VGH/VGL) of the fourth scanning signal lineEG remains as a low level voltage (amplitude VGL), and the fifthswitching element EMT11 is maintained in an OFF state. In this way, thedrain of the drive transistor DRT11 converges to a voltage obtained byreducing the threshold voltage from the first voltage PVDD_H(PVDD_H−Vth). Since a data voltage Vsig1 is applied from the firstsignal line VS to the storage capacitor element CS11, a voltage betweenboth terminals of the storage capacitor element CS11 becomesVsig1−(PVDD_H−Vth). This operation is also the same for the second pixel104_12 which is provided with the same first scanning signal line TG1,the second switching element ICT21 is turned OFF<the third switchingelement ICT22 is turned ON, and a data voltage Vsig1 is applied from thefirst signal line VS2 to one terminal of the storage capacitor elementCS12. That is, the first pixel 104_11 is supplied with a data signalfrom the first signal line VS1 and the second pixel 104_12 is suppliedwith a data signal from the first signal line VS2. Apart from this, thefirst pixel 104_11 and second pixel 104_12 perform the same operations.

Following this, in the first pixel 104_11, a control signal (amplitudeVGH/VGL) of the first scanning signal line TG1 becomes a low levelvoltage (amplitude VGL), and the first switching element TC11 turnedOFF. The same is true for the second pixel 104_12.

Next, with respect to the third pixel 104_21, a control signal(amplitude VGH/VGL) of the first scanning signal line TG2 becomes a highlevel voltage (amplitude VGH) from a low level, and the first switchingelement TCT21 is turned ON. In addition, the first voltage PVDD_H isapplied from the first power supply line PVH to the drive transistorDRT21 the same as described above, a data voltage Vsig2 is applied fromthe first signal line VS1 to the storage capacitor element CS21, and thevoltage between the terminals of the storage capacitor element CS21becomes Vsig2−(PVDD_H−Vth). In the fourth pixel 104_22, a data voltageVsig2 is applied from the first signal line VS2, and the voltage betweenthe terminals of the storage capacitor element CS22 becomesVsig2−(PVDD_H−Vth).

In this way, by switching the ON timing of the first switching elementTG1 and first switching element TG2 in a signal writing and offsetcancel time period, it is possible to write a necessary data voltage toeach pixel together with switching a signal of a signal line from a datavoltage Vsig1 to Vsig2 even when the first signal line VS and secondsignal line VR are shared between pixels 104 in a column direction.

In the first pixel 104_11 in a light emitting time period, a controlsignal (amplitude VGH/VGL) of the first scanning signal line TG1 changesto low level voltage (amplitude VGL) from a high level, and the firstswitching element TCT11 is turned OFF. A control signal (amplitudeVGH/VGL) of the second scanning signal line IG changes to a low levelvoltage (amplitude VGL) from a high level, the second switching elementICT11 is turned OFF, the third switching element ICT21 is turned ON, anda reference voltage Vref is applied to one terminal of the storagecapacitor element CS11. In this way, the voltage of the storagecapacitor element CS11 becomes Vref−Vsig1+(PVDD_H−Vth). A control signal(amplitude VGH/VGL) of the third scanning signal line BG is a high levelvoltage (amplitude VGH), the fourth switching element BCT3 is turned ON,and the sixth switching element BCT2 is turned OFF. In this way, thefirst voltage PVDD_H is applied from the first power supply line PVH tothe source of the drive transistor DRT11. In addition, a control signal(amplitude VGH/VGL) of the fourth scanning signal line EG changes to ahigh level voltage (amplitude VGH) from a low level, and the fifthswitching element EMT11 is turned ON. In this way, a drain currentcontrolled by the gate voltage of the drive transistor DRT11 flows tothe light emitting element EMD11 and light is emitted. Since the gatevoltage of the drive transistor DRT11 is substantially Vref-Fsig1, anyeffects of a threshold voltage Vth are cancelled. In this way, it ispossible to provide a display without receiving any effects of avariation in a threshold voltage of the drive transistor DRT11. Theoperation in a light emitting time period is the same for the secondpixel 104_12. In the third pixel 104_21 and fourth pixel 104_22, thegate voltage of the drive transistor DRT11 is essentially the sameexcept for Vref−Vsig2.

As was explained while referring to FIG. 11, according to one embodimentof the present invention, it is possible to share the fourth switchingelement BCT3 which controls a connection with the first power supplyline PVH which supplies the first voltage PVDD_H among a plurality ofpixels, and it is possible to reduce the number of switching elements(transistors) necessary for forming a pixel circuit. In addition, it ispossible to share the second switching element ICT1 and third switchingelement ICT2 which are used when writing a reference signal and datasignal to a storage capacitor element among a plurality of pixels, andit is possible to reduce the number of switching elements (transistors)necessary for forming a pixel circuit. That is, as is shown in FIG. 12,by making the timing of an initial rise of two first operation signallines TG1, TG2 different, it is possible to write data signals insequence from the first signal line VS1 to the first pixel 104_11 andthird pixel 104_21 adjacent in a column direction. In this case, sinceit is possible to share the second switching element ICT11 and thirdswitching element ICT21, and fourth switching element BCT3 which selecta connection of the first signal line VS1 and second signal line VR1between these two pixels, it is possible to reduce the number ofswitching elements necessary for forming a pixel circuit.

Furthermore, in the structure of the pixel shown in FIG. 11, asexplained as modified example 1, the seventh switching element BCT1 maybe connected in parallel and the first voltage PVDD_H may be applied tothe third power supply line PVD. In this way, it is possible to obtainthe same operational effects as in the modified example 1.

In addition, in the structure of the pixel shown in FIG. 11, asexplained as modified example 1, a three level control signal (amplitudeVGH/VGL1/VGL2) may be applied to the first scanning signal line TG1 andfirst scanning signal line TG2 to control the first switching elementTOT. In this way, it is possible to obtain the same operational effectsas in the modified example 2.

4. Structure of a Pixel

FIG. 13 shows a cross-sectional diagram of a part of a structure of apixel that can be applied to one embodiment of the present invention.FIG. 13 shows an example of a drive transistor DRT and light emittingelement EMD. The drive transistor DRT is arranged in a first substrate110. The drive transistor DRT is formed including a semiconductor layer114, gate insulation layer 116 and gate electrode 118. The semiconductorlayer 114 of the drive transistor DRT is formed from an amorphous orpolycrystalline silicon semiconductor or oxide semiconductor using thesemiconductor properties of a metal oxide. The drive transistor DRT isformed with a channel in a region where the semiconductor layer 114overlaps with the gate electrode 118, and a source region and drainregion are arranged so as to sandwich the channel.

A source electrode 120 and drain electrode 122 are arranged sandwichinga first interlayer insulation layer 124. The source electrode 120 anddrain electrode 122 are each connected to a source region and a drainregion of the semiconductor layer 114 through a contact hole formed inthe first interlayer insulation layer 124 and gate insulation layer 116.A second interlayer insulation layer 126 is arranged above the sourceelectrode 120 and drain electrode 122.

The light emitting element EMD includes a pixel electrode 128, lightemitting layer 130 and opposing electrode 132. In one embodiment of thepresent invention, the pixel electrode 128 is an anode and the opposingelectrode is a cathode. A bank layer 134 is arranged so as to enclosethe pixel electrode 128. The light emitting layer 130 is arranged fromthe pixel electrode 128 to the bank layer 134. The light emitting layer130 includes a light emitting material such as a low molecular or highmolecular organic electroluminescence material. In the case where a lowmolecular organic material is used as the light emitting material, inaddition to the light emitting layer including an organic material withlight emitting properties, a hole injection layer and electron injectionlayer, furthermore, a hole transport layer and electron transport layermay be included to sandwich the light emitting layer 130. For example,the light emitting layer 130 includes a structure in which a layerincluding a light emitting material is sandwiched by a hole injectionlayer and electron injection layer. Furthermore, in addition to a holeinjection layer and electron injection layer, the light emitting layer130 may also be appropriately added with a hole transport layer, anelectron transport layer, a hole block layer, and an electron blocklayer.

Furthermore, in one embodiment of the present invention, the lightemitting element EMD may include what is called a top emission typestructure in which light emitted by the light emitting layer 130 isirradiated to the opposing electrode 132 side. In this case, the pixelelectrode 128 is preferred to be formed from a metal film with highreflectance or a stacked film including such a metal film sin order toreflect the light emitted by the light emitting layer 130 to theopposing electrode side. In a top emission type pixel, light is emittedfrom a surface of the reflecting side to a surface of a side arrangedwith a transistor of a pixel circuit in the light emitting element EMD.As a result, it is possible to form a pixel with a high aperture ratiowithout receiving the effects of the arrangement a transistor arrangedin a pixel.

In the case where the light emitting layer 130 is stacked in sequencewith a hole injection layer, light emitting layer, and electroninjection layer, it is preferred to use ITO (Indium Tin Oxide) which hasexcellent hole injection properties for the pixel electrode 128. ITO isone type of translucent conductive material and while it has a hightransparency to the visible light band, it has very low reflectance. Asa result, a stacked layer structure may be applied of a translucentconductive material and light reflecting layer represented by ITO or IZO(Indium Zinc Oxide) in order to add a function for reflecting light tothe pixel electrode 128. A light reflecting film is preferred to beformed using aluminum (Al) or silver (Ag) or an alloy material orcompound material of aluminum (Al) or silver (Ag). For example, an alloymaterial or compound material in which a few atomic percent of titanium(Ti) is added to aluminum (Al) may be used as the light reflecting film.Since these metal materials have high reflectance with respect to lightin the visible light band, it is possible to increase the amount ofreflected light emitted to the pixel electrode 128 from the lightemitting layer 130. Furthermore, the light reflecting film is notlimited to these metals and apart from the metal material mentionedabove, titanium (Ti), nickel (Ni), molybdenum (Mo) and chrome (Cr) mayalso be used.

A sealing layer 136 is arranged in an upper layer of the light emittingelement EMD. Although the sealing layer 136 is not limited, aninsulation layer formed from an inorganic insulation material and aninsulation layer formed from an organic resin layer may be stacked. Thesealing layer 136 covers the light emitting element EMD and is arrangedin order to prevent the infiltration of water and the like. In the caseof the top emission type structure shown in FIG. 13, it is preferredthat translucency is provided using a cover film such as silicon nitrideor aluminum oxide as the sealing layer 136. In addition, a secondsubstrate may be arranged in an upper part of the sealing layer 136 anda filler material may be arranged therebetween.

As explained above, according to one embodiment of the presentinvention, in a signal writing and offset cancel time period, since afifth switching element EMT is turned OFF while a first voltage PVDD_Hwhich is high voltage is applied and the threshold voltage of a drivetransistor is compensated, it is possible to provide a high margin to anoffset cancel operation. Furthermore, since the fifth switching elementEMT is turned OFF, in an offset cancel time period, a large current doesnot flow via a power supply line, the effects of a drop in voltage dueto wire resistance are relieved, and it is possible to reduce variationin an offset cancel operation in a pixel region 102. In addition, at thetime of an offset cancel operation, by applying the first voltage PVDD_Hwhich is a high voltage from power lines arranged in a mesh shape in arow direction and column direction, the effects of a drop in voltage dueto wire resistance are relieved, and it is possible to make a luminositydistribution uniform particularly in a horizontal direction.Furthermore, by adding a control signal of a switching element forcontrolling a connection between a drain and gate of a drive transistorto a high level, low level two value voltage signal, and driving thefirst switching element with waveforms of three levels added with asecond low level state higher than a low level, it is possible toprevent the generation of light spots in a light emitting period whilecompensating for a threshold voltage of a drive transistor.

What is claimed is:
 1. A display device comprising: a pixel including adrive transistor, a first switching element, a storage capacitor, asecond switching element, a third switching element, a fourth switchingelement, a light emitting element, a fifth switching element and a sixthswitching element; the drive transistor including a first terminal, asecond terminal, and a gate which controlling a current flowing betweenthe first terminal and the second terminals; the first switching elementcontrolling a connection between the second terminal and the gate of thedrive transistor; the storage capacitor including a first terminal and asecond terminal, the second terminal of the storage capacitor connectedto the gate of the driving transistor; the second switching elementcontrolling a connection between the first terminal of the storagecapacitor and a first signal line supplied with a data signal; the thirdswitching element arranged in parallel with the second switching elementand controlling a connection between the first terminal of the storagecapacitor and a second signal line supplied with a reference signal; thefourth switching element controlling a connection between the firstterminal of the drive transistor and a first power supply line suppliedwith a first voltage; the light emitting element including a firstterminal and a second terminal, the first terminal of the light emittingelement connected with the second terminal of the drive transistor andthe second terminal of the light emitting element connected with asecond power supply line supplied with a second voltage lower than thefirst voltage; the fifth switching element controlling a connectionbetween the second terminal of the drive transistor and the firstterminal of the light emitting element; and the sixth switching elementcontrolling a connection between the first terminal of the drivetransistor and a third power supply line supplied with a third voltagelower than the first voltage and higher than the second voltage.
 2. Thedisplay device according to claim 1, farther comprising: a seventhswitching element, the seventh switching element controlling aconnection between the first terminal of the drive transistor and afirst power supply line, and the sixth switching element and the seventhswitching element is operated exclusively.
 3. The display deviceaccording to claim 2, wherein the first power supply line is arranged ina column direction and the third power supply line is arranged in a rowdirection.
 4. The display device according to claim 1, furthercomprising: a reset time period during which the first switching elementand the fifth switching element are turned ON, the fourth switchingelement is turned OFF, a reference voltage is applied to the storagecapacitor from the second signal line via the third switching element,and a third voltage is applied to the first terminal of the drivetransistor from the third power supply line; a signal programming andoffset cancel time period during which the first switching element andthe fourth switching element are turned ON, the fifth switching elementis turned OFF, a voltage based on a data signal from the first signalline is applied to the first terminal of the storage capacitor via thesecond switching element, and the first voltage is applied from thefirst power supply line to the first terminal of the drive transistor;and a light emitting time period during which the first switchingelement is turned OFF, the fourth switching element and the fifthswitching element are turned ON, a reference voltage is applied to thefirst terminal of the storage capacitor via the third switching element,and the light emitting element emits light by a current flowing from thefirst power supply line via the drive transistor.
 5. The display deviceaccording to claim 2, further comprising: a reset time period duringwhich the first switching element and the fifth switching element areturned ON, the fourth switching element is turned OFF, a referencevoltage is applied to the storage capacitor from the second signal linevia the third switching element, and a third voltage is applied to thefirst terminal of the drive transistor from the third power supply line;a signal programming and offset cancel time period during which thefirst switching element and the fourth switching element are turned ON,the fifth switching element is turned OFF, a voltage based on a datasignal from the first signal line is applied to the first terminal ofthe storage capacitor via the second switching element, and the firstvoltage is applied from the first power supply line and the third powersupply line to the first terminal of the drive transistor; and a lightemitting time period during which the first switching element is turnedOFF, the fourth switching element and the fifth switching element areturned ON, a reference voltage is applied to the first terminal of thestorage capacitor via the third switching element, and the lightemitting element emits light by a current flowing from the first powersupply line via the drive transistor.
 6. The display device according toclaim 4, wherein the first switching element is a transistor, and acontrol signal including a first voltage level turning the firstswitching element ON, a second voltage level turning the first switchingelement OFF and a third voltage level higher than the second voltagelevel is applied to a gate of the first switching element as transistor,and in at least the light emitting period, the first switching elementis maintained in an OFF state by a control signal at the third voltagelevel.
 7. The display device according to claim 1, wherein a pluralityof the pixels is arranged and the fourth switching element is sharedamong adjacent pixels.
 8. The display device according to claim 1,wherein a plurality of the pixels is arranged and the second switchingelement and the third switching element are shared among adjacentpixels.
 9. The display device according to claim 1, wherein the firstswitching element, the second switching element and the fifth switchingelement are arranged in a first conductivity type transistor, and thethird switching element and the fourth switching element are arranged ina second conductivity type transistor, the second conductivity type isopposite to the first conductivity type.
 10. The display deviceaccording to claim 2, wherein the first switching element, the secondswitching element, the fifth switching element and the sixth switchingelement are arranged in a first conductivity type transistor, and thethird switching element, the fourth switching element and the seventhswitching element are arranged in a second conductivity type transistor,the second conductivity type is opposite to the first conduction type.11. A method for driving a display device, the display devicecomprising; a pixel including a drive transistor, a first switchingelement, a storage capacitor, a second switching element, a thirdswitching element, a fourth switching element, a light emitting element,a fifth switching element and a sixth switching element; the drivetransistor including a first terminal, a second terminal and a gatewhich controlling a current flowing between the first terminal and thesecond terminals; the first switching element controlling a connectionbetween the second terminal and the gate of the drive transistor; thestorage capacitor including a first terminal and a second terminal, thesecond terminal of the storage capacitor connected to the gate of thedriving transistor; the second switching element controlling aconnection between the first terminal of the storage capacitor and afirst signal line supplied with a data signal; the third switchingelement arranged in parallel with the second switching element andcontrolling a connection between the first terminal of the storagecapacitor and a second signal line supplied with a reference signal; thefourth switching element controlling a connection between the firstterminal of the drive transistor and a first power supply line suppliedwith a first voltage; the light emitting element including a firstterminal and a second terminal, the first terminal of the light emittingelement connected with the second terminal of the drive transistor andthe second terminal of the light emitting element connected with asecond power supply line supplied with a second voltage lower than thefirst voltage; and the fifth switching element controlling a connectionbetween the second terminal of the drive transistor and the firstterminal of the light emitting element; the sixth switching elementcontrolling a connection between the first terminal of the drivetransistor and a third power supply line supplied with a third voltagelower than the first voltage and higher than the second voltage, themethod comprising: in a reset time period the first switching elementand the fifth switching element are turned ON, the fourth switchingelement is turned OFF, a reference voltage is applied to the storagecapacitor from the second signal line via the third switching element,and a third voltage is applied to the other input/output terminal of thedrive transistor from the third power supply line; in a signalprogramming and offset cancel time period the first switching elementand the fourth switching element are turned ON, the fifth switchingelement is turned OFF, a voltage based on a data signal from the firstsignal line is applied to the other terminal of the storage capacitorvia the second switching element, and the first voltage is applied fromthe first power supply line and the third power supply line to the firstterminal of the drive transistor; and in a light emitting time periodthe first switching element is turned OFF, the fourth switching elementand the fifth switching element are turned ON, a reference voltage isapplied to the first terminal of the storage capacitor via the thirdswitching element, and the light emitting element emits light by acurrent flowing from the first power supply line via the drivetransistor.
 12. The method of driving a display device according toclaim 11, farther comprising a seventh switching element, the seventhswitching element controlling a connection between the first terminal ofthe drive transistor and a first power supply line, and the sixthswitching element and the seventh switching element is operatedexclusively, in the signal programming and offset cancel time period,the first voltage is applied to the first terminal of the drivetransistor from the first power supply line.
 13. The method of driving adisplay device according to claim 12, wherein the first power supplyline is arranged in a column direction, the third power supply line isarranged in a row direction, and the first voltage is applied to thefirst terminal of the drive transistor from the first power supply lineand the third power supply line.
 14. The method of driving a displaydevice according to claim 11, wherein in the light emitting time period,the first switching element is applied with a control signal of a thirdvoltage level lower than the first voltage level turning the firstswitching element ON and higher than the second voltage level turningthe first switching element OFF.
 15. A display device comprising: afirst pixel, a second pixel, a third pixel, and fourth pixel, each ofthe first to the fourth pixel including a drive transistor, a firstswitching element, a storage capacitor, a second switching element, athird switching element, a fourth switching element, a light emittingelement and a fifth switching element; the drive transistor arrangedincluding a first terminal, a second terminal and a gate whichcontrolling a current flowing between the first terminal and the secondterminals; the first switching element controlling a connection betweenthe second terminals and the gate of the drive transistor; the storagecapacitor including a first terminal and a second terminal, the secondterminal of the storage capacitor connected to the gate of the drivingtransistor; the second switching element controlling a connectionbetween the first terminal of the storage capacitor and a first signalline supplied with a data signal; the third switching element arrangedin parallel with the second switching element and controlling aconnection between the first terminal of the storage capacitor and asecond signal line supplied with a reference signal; the fourthswitching element controlling a connection between the first terminal ofthe drive transistor and a first power supply line supplied with a firstvoltage; the light emitting element including a first terminal and asecond terminal, the first terminal of the light emitting elementconnected with the second terminal of the drive transistor and thesecond terminal of the light emitting element connected with a secondpower supply line supplied with a second voltage lower than the firstvoltage; and the fifth switching element controlling a connectionbetween the second terminal of the drive transistor and the firstterminal of the light emitting element; wherein each of the drivetransistor included in the first to the fourth pixels connected to athird power supply line connected with a sixth switching elementcontrolling the application of a third voltage lower than the firstvoltage and higher than the second voltage to the first terminal, thefirst and the second pixel, and the third and the fourth pixel arearranged in a row direction, the first and the third pixel, and thesecond and the fourth pixel are arranged in a column direction, thefourth switching element is shared between the first to fourth pixels,and the second switching element and the third switching element areshared between the first and third pixels, and the second and fourthpixels arranged in a column direction.
 16. The display device accordingto claim 15, further comprising: a reset time period during which ineach of the first to fourth pixels, the first switching element and thefifth switching element are turned ON, the fourth switching element isturned OFF, a reference voltage is applied to the storage capacitor fromthe second signal line via the third switching element, and a thirdvoltage is applied to the first terminal of the drive transistor fromthe third power supply line; a first signal programming and offsetcancel time period during which in each of the first pixel and thesecond pixel, the first switching element and the fourth switchingelement are turned ON, the fifth switching element is turned OFF, avoltage based on a data signal from the first signal line is applied tothe first terminal of the storage capacitor via the second switchingelement, the first voltage is applied from the first power supply lineto the first terminal of the drive transistor, and in the third pixeland the fourth pixel, the first switching element is OFF; a secondsignal programming and offset cancel time period during which in each ofthe third pixel and the fourth pixel, the first switching element andthe fourth switching element are turned ON, the fifth switching elementis turned OFF, a voltage based on a data signal from the first signalline is applied to the first terminal of the storage capacitor via thesecond switching element, the first voltage is applied from the firstpower supply line to the first terminal of the drive transistor, and inthe first pixel and the second pixel, the first switching element isOFF; and a light emitting time period during which n each of the firstto fourth pixels, the first switching element is turned OFF, the fourthswitching element and the fifth switching element are turned ON, areference voltage is applied to the first terminal of the storagecapacitor via the third switching element, and the light emittingelement emits light by a current flowing from the first power supplyline via the drive transistor.
 17. The display device according to claim16, further comprising: a waiting time period during which in each ofthe first to fourth pixels, the first switching element is turned OFF,the second switching element and the third switching element aremaintained in an ON state, a reference voltage is applied from thesecond signal line to the storage capacitor via the third switchingelement, the fifth switching element changes from ON to OFF, next thefourth switching element changes from OFF to ON, the sixth switchingelement changes from ON to OFF, and the first voltage is applied to thefirst terminal of the drive transistor, the waiting time period duringbetween the reset time period and the first and second signalprogramming and offset cancel time periods.